1. Field of the Invention
The invention relates to a method and molding device for producing molded blanks of ceramic powder, for instance nuclear reactor fuels such as UO.sub.2, PuO.sub.2, ThO.sub.2 or (U/Pu)O.sub.2 -mixed oxides in powder form, in a mold which belongs to the molding press and comprises a die with a hole. In this hole a plunger with a head is displaceable relative to this die. The wall of the hole is lubricated by means of a lubricant by the relative movement of the head of the plunger in the hole.
2. Description of the Prior Art
Such a method and such a molding device are known from German Pat. No. 1 920 308. The device according to this patent is equipped with a lower plunger having a head which has at the outermost end of the plunger, a pressure plate with a pressure surface for the ceramic powder. This head further has a guide part adapted to the cross section of the hole, which is arranged on the side of the pressure plate facing away from the pressure surface at a distance from this pressure plate at the lower plunger. Between the pressure plate and the guide part, the head has an intermediate member which has a smaller cross section than the pressure plate and the guide part and has radial holes which are connected to an axial lubrication canal in the lower plunger.
In such a molding device it is not necessary to add a lubricant, usually organic, to ceramic powder prior to pressing into molded blanks. This lubricant would have to be particularly uniformly distributed in the ceramic powder and, in addition, would have to be removed from these molded blanks before the molded blanks are sintered, by a separate dewaxing anneal.
In the known molding device, the friction between the ceramic powder and the molding tool, particularly the wall of the die hole, is reduced considerably during the pressing of the blanks by a thin film of lubricant on the wall of the die hole. This film of lubricant is formed by an oil as the lubricant which is liquid at the operating temperature of the mold formed of the die and the plunger. This oil is pumped through the axial canal in the lower plunger to the openings of the radial canals in the intermediate member and at the same time, a relative motion between the die and the lower plunger is carried out, for instance by ejecting a molded blank from the hole of the die. The diameters of the pressure plate at the lower plunger are chosen so that they allow the development of a film of lubricant consisting of liquid oil, for instance 10 to 20 .mu.m thick, on the wall of the die hole.
An important advantage of the known device is that it can generate a uniform lubricant film on the wall of the die hole. Such a uniform film of lubricant on the wall of the die cavity, which is not torn off in any area, is highly important for avoiding different local densities in the molded part, which lead, during the sintering of the molded blank, to deviations from the desired shape or, to a greater extent, to considerable rejects due to chipping off and cracks, and which are caused by different heavy friction occurring in an area at the wall of the die cavity during pressing, where the lubricant film is broken. Unfortunately, conditions of operation occur in which the lubricant film is broken or inadequate causing production of defective blanks and their rejection.